Catalyst based on palladium, gold, alkali, and lanthanoid, and a method for producing vinyl acetate

ABSTRACT

The invention relates to a catalyst which contains palladium and/or compounds thereof, gold and/or compounds thereof, alkali metal compounds and at least one lanthanoid metal and/or compounds thereof. The invention also relates to the utilization of the catalyst in order to produce vinyl acetate from acetic acid, ethylene and oxygen or gases containing oxygen.

This application is a 371 of PCT/EP98/027816 filed Dec. 2, 1998.

The present invention relates to a catalyst which comprises palladiumand/or its compounds, gold and/or its compounds, alkali metal compoundsand at least one lanthanoid metal and/or its compounds, and to its usefor preparing vinyl acetate from acetic acid, ethylene and oxygen oroxygen-containing gases.

It is known that ethylene can be converted in the gas phase with aceticacid and oxygen or oxygen-containing gases on palladium/gold/alkalimetal-containing fixed bed catalysts into vinyl acetate.

The palladium/gold/alkali metal-containing catalysts have a particularnoble metal distribution, with the noble metals being present in a shellon the carrier particles, while the core of the particles issubstantially free of noble metals. Catalysts with this noble metaldistribution are distinguished by an increased specific productivity (gof vinyl acetate/g of noble metal). The noble metal compound in shellform is achieved by impregnation and subsequent precipitation of thenoble metals using alkaline compounds.

The process disclosed in U.S. Pat. No. 4,048,096 for preparingpalladium, potassium and gold-containing catalysts entails initialimpregnation of the carrier material with an aqueous solution whichcomprises a mixture of palladium and gold salts. The metal salts arethen converted by treatment with alkalis into water-insoluble compoundsand are fixed on the carrier material in this way. Subsequent treatmentwith a reducing agent reduces the palladium and gold compounds to thecorresponding metals. Finally, the carrier material loaded withpalladium and gold is treated with an alkali metal acetate solution anddried. The impregnation step with the aqueous solution containingpalladium and gold salts is characterized by the volume of theimpregnation solution corresponding to the pore volume of the carriermaterial. The resulting catalyst has a shell structure in whichpalladium and gels are dispersed in a shell thickness of about 0.5millimeter over the surface of the carrier material.

U.S. Pat. No. 3,775,342 also discloses a process for preparingpalladium, potassium and gold-containing catalysts by impregnation witha solution of palladium and gold salts, by subsequent treatment with analkali solution, which results in water-insoluble palladium and goldcompounds precipitating on the carrier, and by subsequent reduction ofthe metal compounds to the corresponding noble metals. Treatment of thecarrier material with an alkali metal acetate solution can take placebefore or after the reduction step.

U.S. Pat. No. 5,185,308 discloses a palladium, potassium andgold-containing shell catalyst in which the noble metals are dispersedin a shell thickness of 1 millimeter over the carrier material. Theknown catalyst has a ratio of gold to palladium in the range from 0.6 to1.25 by weight.

It is further known to prepare a palladium, potassium andgold-containing shell catalyst by washing a carrier material, which hasbeen provided with a binder, for example an alkali metal or alkalineearth metal carboxylate, before the impregnation with an acid, andtreating with a base after the impregnation (EP-A-0 519 435).

In the process disclosed in U.S. Pat. No. 5,332,710 for preparing apalladium, gold and potassium-containing shell catalyst, the carrierimpregnated with an aqueous palladium and gold salt solution is immersedin an aqueous fixing solution containing sodium hydroxide or potassiumhydroxide and agitated therein for at least 0.5 h.

It has now been found, surprisingly, that catalysts of this type can bedistinctly improved by adding at least one lanthanoid metal and/or alanthanoid metal compound, i.e. provide a higher space-time yield withidentical or higher selectivity for vinyl acetate.

The invention accordingly relates firstly to a process for preparingvinyl acetate in the gas phase from ethylene, acetic acid and oxygen oroxygen-containing gases on a catalyst which comprises 0.5-2.0% by weightof palladium and/or its compounds, 0.2-1.3% by weight of gold and/or itscompounds, and 0.3-10% by weight of alkali metal compounds on a carrier,wherein the catalyst additionally comprises 0.01-1% by weight of atleast one lanthanoid metal and/or its compounds, the percentagesrelating to the metal contents, based on the total mass of the catalyst.

The invention secondly relates to a catalyst which comprises 0.5-2.0% byweight of palladium and/or its compounds, 0.2-1.3% by weight of goldand/or its compounds, and 0.3-10% by weight of alkali metal compounds ona carrier, wherein the catalyst additionally comprises 0.01-1% by weightof at least one lanthanoid metal and/or its compounds, the percentagesrelating to the metal contents, based on the total mass of the catalyst.

The procedure for preparing the catalysts according to the invention ispreferably as follows (U.S. Pat. Nos. 3,775,342, 4,048,096, 5,332,710):

(1) First the carrier particles are impregnated one or more times bybeing intimately mixed with at least one solution of at least one saltof the elements palladium and gold, and of at least one salt of at leastone lanthanoid metal.

(2) The pretreated carrier is treated with a fixing solution with analkaline reaction, which results in the noble metals and the lanthanoidmetals being precipitated in the form of water-insoluble compounds onthe surface of the carrier particles, and thus being fixed.

(3) The noble metal compounds deposited on the carrier particles arereduced to the corresponding metals by treatment with a reducing agent.A noble metal shell doped with at least one lanthanoid metal is producedin this way on the surface of the carrier particles.

(4) Interfering anions are removed by washing the treated catalyst.

(5) The treated catalyst is dried at not above 150° C.

(6) The dried carrier is treated with a solution which contains at leastone alkali metal compound.

(7) Finally, the treated carrier is dried at not above 150° C.

The procedure in step (1) can also be to apply the salt solutionscontaining catalytically active substances to the carrier by single ormultiple spraying on, vapor deposition or immersion.

The term “lanthanoid metals” means the 14 rare earth elements cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium,and the elements scandium, yttrium and lanthanum because their chemicalbehavior resembles that of the rare earth elements.

Suitable carriers are the known inert carrier materials such as silica,alumina, aluminosilicates, silicates, titanium oxide, zirconium oxide,titanates, silicon carbide and carbon. Particularly suitable carriers ofthis type are those with a specific surface area of 40 to 350 m²/g(measured by the BET method) and an average pore radius of 50 to 2000 Å(Angstrom) (measured by mercury porosimetry), especially silica (SiO₂)and SiO₂/Al₂O₃ mixtures. These carriers can be used in any form such as,for example, in the form of beads, tablets, rings, stars or particles ofother shapes, with a diameter or length and thickness generally of 3 to9 mm.

Carriers of these types can be prepared, for example, from aerogenicSiO₂ or an aerogenic SiO₂/Al₂O₃ mixture which can be prepared, forexample, by flash hydrolysis of silicon tetrachloride or a silicontetrachloride/-aluminum trichloride mixture in an oxyhydrogen flame(U.S. Pat. No. 3,939,199).

Suitable solvents for the palladium, gold, alkali metal and lanthanoidmetal salts are all compounds in which the selected salts are solubleand which can easily be removed again after the impregnation by drying.Suitable for the acetates are, in particular, unsubstituted carboxylicacids having 2 to 10 carbon atoms such as acetic acid, propionic acid,n- and iso-butyric acid and the various valeric acids. Among thecarboxylic acids, acetic acid is preferred because of its physicalproperties and also for economic reasons. Water is particularly suitablefor the chlorides and chloro and acetato complexes. Additional use ofanother solvent is expedient if the salts are insufficiently soluble inacetic acid or in water. Thus, for example, palladium chloride can bedissolved considerably better in an aqueous acetic acid than in glacialacetic acid. Suitable additional solvents are those which are inert andare miscible with acetic acid or water. Those which may be mentioned asadditions for acetic acid are ketones such as acetone and acetylacetone,also ethers such as tetrahydrofuran or dioxane, but also hydro-carbonssuch as benzene.

It is possible to apply a plurality of salts of palladium, gold, alkalimetal and the particular lanthanoid metal, but generally exactly onesalt of each of these elements is applied.

The elements palladium and gold which are to be applied in each case inthe procedure of step (1), and the lanthanoid metal to be applied ineach case, can be applied in the form of salt solutions, singly or elsein any suitable combination in any suitable sequence, preferably using asingle solution which contains these elements to be applied in the formof salts. It is particularly preferred to use a single solution whichcontains exactly one salt of each of these elements to be applied.

This solution preferably contains a salt of a single lanthanoid metal,but it is also possible to use a solution which contains one salt ofeach of different lanthanoid metals.

Where the following speaks generally of “the solution of the salts”, thesame applies analogously to the case where a plurality of solutions areemployed in sequence, each of which contains only part of the totalityof salts to be applied, in which case the total of the individual partsamounts to the total quantity of the salts to be applied to the carrier.

For the procedure of step (1), the solution of the salts is applied tothe carrier particles by impregnating the latter one or more times withthis solution, employing the total volume of the solution all at once ordivided into two or more part-volumes. However, it is expedient to usethe total volume of the salt solution all at once, so that the carrierparticles are impregnated with the required amount of elements to beapplied by a single impregnation, in which case drying can followimmediately. In the case of impregnation sequentially with a pluralityof part-volumes, drying is carried out immediately after eachimpregnation.

“Immediate” drying means in this connection that drying the impregnatedparticles must start without delay. It is generally sufficient for thiscase to start drying the particles no later than half an hour after theend of an impregnation.

The impregnation of the carrier particles with the solution of the saltsto be applied is carried out by covering the carrier particles with thesolution and, where appropriate, then pouring off or filtering offexcess solution. It is advantageous, with regard to losses of solution,to employ only the quantity of solution corresponding to the integralpore volume of the catalyst carrier.

It is expedient to mix the carrier particles intimately during theimpregnation, for example in a rotating or agitated flask or a mixingdrum, in which case drying can follow immediately. The speed of rotationor intensity of the agitation must, on the one hand, be sufficient toensure good mixing and wetting of the carrier particles but must, on theother hand, not be so great that there is considerable abrasion of thecarrier material.

The solution of the salts should have a temperature which is high enoughto prevent the salts precipitating during the application to thecarrier. The temperature should, however, generally not be much above70° C. in order to avoid excessive evaporation of the solvent anddecomposition of the noble metal compounds.

The treatment of the carrier particles impregnated in step (1) with asolution with an alkaline reaction converts the salts of the appliedelements into water-insoluble compounds, and they are thus fixed to thesurface of the carrier (step (2)).

Examples of fixing solutions which can be used are aqueous solutionswith an alkaline reaction. Examples of such solutions are aqueoussolutions of alkali metal silicates, alkali metal carbonates andbicarbonates or alkali metal hydroxides.

An aqueous solution of the alkali metal hydroxides, in particularpotassium or sodium hydroxide, is preferred. Aqueous solutions whichcontain boron compounds can also be used as solutions with an alkalinereaction. Particularly suitable in this case are aqueous solutions ofborax, potassium tetraborate or mixtures of alkali metal hydroxidesolution and boric acid. The alkaline solution may have bufferingproperties.

The amount of the compound with an alkaline reaction present in thefixing solution is expediently such that it is at least sufficient forstoichiometric conversion of the applied palladium, gold and lanthanoidmetal salts into water-insoluble compounds.

However, it is also possible to use an excess of the compound with analkaline reaction present in the fixing solution, the excess generallybeing 1 to 10 times the amount required by the stoichiometry.

The volume of the fixing solution must be at least sufficient to coverthe impregnated carrier completely with the fixing solution. The fixingpreferably takes place by the rotation immersion technique disclosed inU.S. Pat. No. 5,332,710, which is incorporated herein by reference. Thistechnique comprises agitating the carrier which is completely covered bythe fixing solution by rotation from the start of the treatment with thefixing solution.

Every type of rotation or similar treatment which keeps the carrierparticles agitated can be used, because the exact manner is notcritical. The intensity of the agitation is important, however. Itshould be sufficient for the entire surface area of the impregnatedcarrier to be wetted uniformly with the alkaline fixing solution.

The treated carrier is then left to stand in the fixing solution at roomtemperature for up to 16 hours in order to ensure that the appliedpalladium, gold and lanthanoid metal salts are completely precipitatedin the form of water-insoluble compounds on the catalyst carrier.

The reaction on the carrier can, however, also be carried out atelevated temperature, for example at 70° C.

After the fixation is complete, the supernatant fixing solution ispoured away. This can be followed, where appropriate, by washing thetreated carrier in order to remove the soluble compounds present on thetreated carrier, for example the alkali metal chlorides liberated in thefixing step and any excess which is present of the compound with analkaline reaction present in the fixing solution, by washing.

For this purpose, the treated carrier is continuously washed with thewashing liquid, preferably with running demineralized water, at roomtemperature. The washing is continued until interfering anions, forexample chlorides, are substantially removed from the carrier.

The moist impregnated catalyst carrier can then be dried, which isexpedient if the subsequent reduction of the deposited noble metalcompounds to the corresponding metals (step (3)) is carried out in thegas phase.

Reduction of the water-insoluble compounds fixed on the catalyst carrierto the corresponding metals can be carried out with a gaseous reducingagent (step (3)).

The reduction temperature is generally between 40 and 260° C.,preferably between 70 and 200° C. It is generally expedient to use forthe reduction a reducing agent which is diluted with inert gas andcontains 0.01 to 50% by volume, preferably 0.5 to 20% by volume, ofreducing agent. It is possible to use as inert gas, for example,nitrogen, carbon dioxide or a noble gas. Examples of suitable reducingagents are hydrogen, methanol, formaldehyde, ethylene, propylene,isobutylene, butylene or other olefins. The reduction can also becarried out in liquid phase at a temperature from 0° C. to 90° C.,preferably from 15 to 25° C. Examples of reducing agents which can beused are aqueous solutions of hydrazine, formic acid or alkali metalborohydrides, preferably sodium borohydride. The amount of reducingagent depends on the amount of the noble metals; the reductionequivalent should be at least equal to oxidation equivalent in quantity,but larger amounts of reducing agent are not harmful.

It is essential to select the reduction conditions in the reduction stepso that the fixed water-insoluble noble metal compounds are reduced tothe corresponding noble metals. It is, on the other hand, immaterialwhether the fixed water-insoluble lanthanoid metal compounds are alsoconverted under the selected reduction conditions into the correspondinglanthanoid metals, because it is not critical for the suitability of thenovel catalysts for preparing vinyl acetate whether the lanthanoidmetals are present as elements and/or their compounds in the noble metalshell of the novel catalysts.

If no washing step takes place after the fixation is complete (step(2)), or if the reduction takes place with an aqueous solution of areducing agent, the treated catalyst carrier must, after the reductionis complete, be washed several times to remove interfering compounds,for example to remove chloride residues derived from the impregnationstep and released due to the fixation and reduction of the noble metals(step (4)).

For this purpose, the treated carrier is washed continuously with thewashing liquid, preferably with running demineralized water, at roomtemperature until interfering anions, for example chlorides, areremoved.

If an aqueous solution of a reducing agent is used in step (3), residuesof the reducing agent used can also be removed with the washing step.

The catalyst is then dried at temperatures not exceeding 150° C. (step(5)).

In step (6), the dried catalyst carrier is then treated, preferablyimpregnated, one or more times with a solution of an alkali metalcompound, the total volume of the solution being employed all at once ordivided into part-volumes. However, it is expedient to use the totalvolume of the solution all at once, so that the carrier particles areimpregnated with the required amounts of alkali metal compound to beapplied by a single impregnation. The volume of the solution of thealkali metal compound is, in the case of single or multipleimpregnation, generally between 60 and 110%, preferably between 80 and100%, of the pore volume.

The solution of the alkali metal compound can also be applied to thecarrier by single or multiple spraying on, vapor deposition orimmersion.

After the treatment with a solution of an alkali metal compound, thecatalyst carrier is finally dried at no higher than 150° C. (step (7)).

The alkali metal compound is used in an amount such that the catalystcarrier contains 0.1 to 10% by weight of alkali metal after the drying.

The drying of the treated catalyst carrier to be carried out in steps(5) and (7) takes place in a stream of hot air or in a stream of inertgas, for example in a stream of nitrogen or carbon dioxide. Thetemperature during this drying should generally be 60 to 150° C.,preferably 100 to 150° C. Drying is moreover carried out, whereappropriate, under reduced pressure, generally from 0.01 MPa to 0.08MPa.

If the drying forms part of step (1) and, where appropriate, the othersteps, the procedure is the same.

The finished shell catalysts containing palladium, gold, alkali metaland at least one lanthanoid metal have the following metal contents:

Palladium content: generally 0.5-2.0% by weight, preferably 0.6-1.5% byweight; Gold content: generally 0.2-1.3% by weight, preferably 0.3-1.1%by weight; Alkali metal content: generally 0.3-10% by weight, andpotassium is preferably used. Potassium content: generally 0.5-4.0% byweight, preferably 1.5-3.0% by weight; Lanthanoid metal content:generally 0.01-1% by weight, preferably 0.05-0.5% by weight.

If more than one lanthanoid metal is used to dope the palladium, goldand alkali metal-containing shell catalysts, the term “lanthanoid metalcontent” means the total content of all the lanthanoid metals present inthe finished catalyst. The stated percentages always relate to theamounts of the elements palladium, gold, alkali metal and lanthanoidmetal present in the catalyst, based on the total mass of the catalyst(active elements plus anions plus carrier material).

Suitable salts are all salts of palladium, gold, an alkali metal and alanthanoid element which are soluble; the acetates, the chlorides, andthe acetato and chloro complexes are preferred. However, in the case ofinterfering anions such as, for example, in the case of chlorides, itmust be ensured that these anions are substantially removed before useof the catalyst. This takes place by washing the doped carrier, forexample with water, after, for example, the palladium and gold whichhave been applied as chloride have been converted into an insolubleform, for example through the fixation with compounds having an alkalinereaction and/or by reduction (steps (2) and (3)).

Particularly suitable salts of palladium and gold are chloride, chlorocomplexes and carboxylates, preferably the salts of aliphaticmonocarboxylic acids having 2 to carbon atoms, for example the acetate,propionate or butyrate. Further suitable examples are the nitrate,nitrite, oxide hydrate, oxalate, acetylacetonate or acetoacetate.Because of the good solubility and availability, preferred palladium andgold salts are in particular the chlorides and chloro complexes ofpalladium and gold.

The alkali metal compound preferably employed is at least one sodium,potassium, rubidium or caesium compound, in particular a potassiumcompound. Particularly suitable compounds are carboxylates, inparticular acetates and propionates. Compounds which are converted underthe reaction conditions into the alkali metal acetate, such as, forexample, the hydroxide, the oxide or the carbonate, are also suitable.

The lanthanoid metal compound employed is preferably at least onepraseodymium, neodymium, samarium, europium or dysprosium compound.However, it is also possible to employ mixtures of these compounds.

The chlorides, nitrates, acetates and acetylacetonates are particularlysuitable as lanthanoid metal compound.

In the novel catalysts, the noble metals and the particular lanthanoidmetals and/or their compounds are applied in a shell on the carrierparticle.

Vinyl acetate is generally prepared by passing acetic acid, ethylene andoxygen-containing gases at temperatures from 100 to 220° C., preferably120 to 200° C., under pressures from 0.1 to 2.5 MPa, preferably 0.1 to2.0 MPa, over the finished catalyst, it being possible to circulateunreacted components. It is also advantageous in some circumstances todilute with inert gases such as nitrogen or carbon dioxide. Carbondioxide is particularly suitable for the dilution because it is formedin small amounts during the reaction.

With the same reaction conditions it is possible with the aid of thenovel catalysts to prepare more vinyl acetate per reactor volume andtime with, at the same time, improved selectivity by comparison withknown catalysts.

This facilitates the workup of the resulting crude vinyl acetate becausethe vinyl acetate content in the gas discharged from the reactor ishigher, which further results in a saving of energy in the workup part.A suitable workup is described, for example, in U.S. Pat. No. 5,066,365.

If, on the other hand, it is wished to keep the spacetime yieldconstant, it is possible to reduce the reaction temperature and thuscarry out the reaction more selectively, with the same totalproductivity, in which case there is a saving of precursors. This isalso associated with a decrease in the amount of carbon dioxide, whichis formed as byproduct and therefore must be removed, and in the loss ofentrained ethylene which is associated with this removal. In addition,this procedure results in an increase in the useful life of thecatalyst.

The following examples are intended to illustrate the invention but donot restrict it. The percentages of the elements palladium, gold,potassium and of the lanthanoid element are percent by weight based onthe total mass of the catalyst.

The catalyst carrier used was the SiO₂ carrier available from Süd-Chemiewith the name KA 160 in the form of beads with a diameter of 5 mm. Thepore volume of 1 l of carrier was 335 ml.

EXAMPLE 1

5.37 g (=0.0164 mol) of potassium tetrachloropalladate, 3.36 g (0.0089mol) of potassium tetrachloroaurate and 0.74 g (0.0018 mol) ofpraseodymium trinitrate pentahydrate were weighed out together anddissolved in 90 ml of demineralized water (solution volume=100% of thepore volume). With gentle agitation, this solution was completelyadsorbed onto 147.5 g of the carrier material at room temperature. Toprecipitate insoluble palladium, gold and praseodymium compounds, whichleads to formation of a noble metal shell, the pretreated carrier wasmixed with a solution of 3.1 g of sodium hydroxide in 300 ml ofdemineralized water. Immediately after addition of the alkaline fixingsolution, the carrier was agitated in a rotary evaporator rotating at arate of 5 revolutions per minute (rpm) for a period of 2.5 hours. Tocomplete the precipitation, the mixture was left to stand at roomtemperature for a period of 14 hours. The supernatant solution was thenpoured off, and the mixture was washed with demineralized water untilfree of chloride. A water flow rate of 200 ml/minute for approximately 5hours was necessary for this. To check for freedom from chloride, asilver nitrate solution was added to the washing water and it wasexamined for silver chloride precipitation. The catalyst wassubsequently dried at a temperature of 100° C. for a period of 2 hours.It was then reduced with a gas mixture consisting of 5% by volumeethylene and 95% by volume nitrogen, passing this gas mixture over thecatalyst at a temperature of 150° C. for a period of 5 hours. Thereduced catalyst was then impregnated with a solution of 10 g ofpotassium acetate in 75 ml of demineralized water (solution volume=83%of the pore volume) in portions and dried with hot air at a temperatureof 100° C. for a period of 2 hours.

The finished catalyst contained 1.1% by weight Pd, 1.1% by weight Au,2.5% by weight K and 0.16% by weight Pr.

EXAMPLE 2

The procedure was analogous to that of Example 1 but the lanthanoidmetal compound used was 0.71 g (0.0017 mol) of samarium trinitratepentahydrate in place of praseodymium trinitrate pentahydrate.

The finished catalyst contained 1.1% by weight Pd, 1.1% by weight Au,2.5% by weight K and 0.16% by weight Sm.

EXAMPLE 3

The procedure was analogous to that of Example 1 but 0.7 g (0.0016 mol)of europium trinitrate pentahydrate was used as lanthanoid metalcompound.

The finished catalyst contained 1.1% by weight Pd, 1.1% by weight Au,2.5% by weight K and 0.15% by weight Eu.

EXAMPLE 4

The procedure was analogous to that of Example 1 but 0.34 g (0.0008 mol)of neodymium trinitrate pentahydrate was used as lanthanoid metalcompound.

The finished catalyst contained 1.1% by weight Pd, 1.1% by weight Au,2.5% by weight K and 0.07% by weight Nd.

EXAMPLE 5

The procedure was analogous to that of Example 1 but 0.3 g (0.0008 mol)of dysprosium trichloride hexahydrate was used as lanthanoid metalcompound.

The finished catalyst contained 1.1% by weight Pd, 1.1% by weight Au,2.5% by weight K and 0.08% by weight Dy.

EXAMPLE 6

The procedure was analogous to that of Example 5 but 0.6 g (0.0016 mol)of dysprosium trichloride hexahydrate was used.

The finished catalyst contained 1.1% by weight Pd, 1.1% by weight Au,2.5% by weight K and 0.16% by weight Dy.

COMPARATIVE EXAMPLE 1a

The procedure was as in Example 1 but no lanthanoid metal salts wereadded to the impregnation solution containing potassiumtetrachloropalladate and potassium tetrachloroaurate.

The finished catalyst contained 1.1% by weight Pd, 1.1% by weight Au and2.5% by weight K.

The novel catalysts prepared as in Examples 1-6, and the known catalystprepared as in Comparative Example 1a, were tested in a Berty reactor.The average temperature of the jacket of the Berty reactor was chosen sothat a constant oxygen conversion of 45% was observed.

The results are to be found in the table.

Space-time CO₂ Example yield selectivity 1 793 8.97 2 780 9.23 3 8028.79 4 726 8.50 5 733 9.0 6 722 9.3 Comparative Example 1a 683 10.9

Space-time yield in gram of vinyl acetate per liter of catalyst andhour.

CO₂ selectivity in % based on the amount of ethylene reacted.

It was found, surprisingly, that even small additions of lanthanoidmetals to the known palladium, gold and potassium-containing catalystsdistinctly improve the CO₂ selectivity and the productivity (space-timeyield) of these catalysts in preparing vinyl acetate.

What is claimed is:
 1. A catalyst comprising 0.2 to 1.3% by weight ofgold or a gold compound calculated as gold metal, 0.3 to 10% by weightof an alkali metal compound calculated as alkali metal, 0.5 to 2.0% byweight of palladium or palladium compound calculated as palladium metaland 0.01 to 1% by weight of at least one lanthanoid compound or metalcalculated as lanthanoid metal on a carrier, the metal percentage basedon the total mass of the catalyst.
 2. A catalyst of claim 1 containingat least one potassium compound.
 3. A catalyst of claim 1 wherein theamount of lanthanoid compound or metal is 0.05 to 0.5% by weightcalculated as lanthanoid metal.
 4. A catalyst of claim 1 wherein thelanthanoid metal is selected from the group consisting of praseodymium,samarium, europium, neodymium and dysprosium.